1. Field of the Invention
This invention relates to a liquid crystal display and more particularly to a liquid crystal display which provides a wide viewing angle and high display quality by controlling the orientation of liquid crystal directors.
2. Description of the Related Art
Liquid crystal displays have the advantages of their small size, slim form, and low power consumption. Therefore, they are becoming increasingly commercially practical for use in office automation machines, audio and visual machines, and the like. Particularly, the liquid crystal displays of the active matrix type using thin-film transistors (TFTs) as switching elements, which can display high-definition dynamic images, are used for television displays, etc.
To form the liquid crystal display, as shown in FIG. 1, a TFT substrate 2 comprising TFTs, display electrodes, etc., disposed like a matrix on a transparent substrate such as glass, and an opposed substrate 4 having common electrodes are affixed with a several .mu.m thick liquid crystal layer 3 between, and both the substrates affixed to each other are sandwiched between two polarization plates 1 and 5 perpendicular to each other in a polarization axis direction.
The TFT substrate 2 has a structure in which TFTs are formed around the intersections of gate lines (scanning lines) and drain lines (data lines) and display electrodes located like a matrix are connected to the TFTs. The gate lines are scanned and selected in order, thereby turning on all TFTs on the same scanning line. Data signals synchronized with the turning on of the TFTs are supplied to the display electrodes on the drain lines. The common electrodes are also set to a predetermined potential in synchronization with gate line scanning, thereby applying a predetermined potential difference to display picture element capacitors formed by the common electrodes and opposite display electrodes for driving the liquid crystal.
Particularly, in the liquid crystal display using the ECB (electrically controlled birefringence) system, voltage is applied to display electrodes and common electrodes for controlling the orientation state of liquid crystal directors and a birefringence change is made in white light incident from a light source to provide an optical switch function. For example, a nematic crystal liquid having negative anisotropy of dielectric constant is used as the liquid crystal layer 3 and the initial orientation of liquid crystal directors is set to the direction vertical to the substrate face; the liquid crystal display of this type is called VAN (vertically aligned nematic) type.
In FIG. 1, white light incident from the TFT substrate 2 is passed through the first polarization plate 1 and results in linear polarization only. When no voltage is applied, the incident linear polarization is not subjected to birefringence in the liquid crystal layer 3, and thus is shut off by the second polarization plate 5 and black is displayed (normally black mode). When a predetermined voltage is applied to the liquid crystal layer 3, the orientation of the liquid crystal directors changes to the direction in which the orientation vector of the liquid crystal molecules having negative dielectric constant anisotropy approach a right angle with the electric field direction. Since a liquid crystal has anisotropy in refractive index, the linear polarization incident on the liquid crystal layer is subjected to birefringence and becomes elliptic polarization and light is transmitted through the second polarization plate 5. The transmitted light strength in the liquid crystal display depends on the voltage applied to the liquid crystal layer. Therefore, gradation display is enabled by adjusting the applied voltage for each picture element; light and dark (monochrome) display at the picture elements is visible as a predetermined display image on the entire display.
In the VAN type, macromolecular films of polyimide (S1Nx) are formed on the surfaces of both substrates 2 and 4 and rubbing treatment is applied to the films, thereby giving a predetermined pretilt angle to the initial vertical orientation of the liquid crystal directors for controlling the orientation of the liquid crystal directors. Further, for example, the opposed substrate 4 is formed with a color filter installed on an optical path and the color capability is combined with the optical shutter effect of ECB to provide color display.
FIG. 2 is a plan view showing a light transmission state when the conventional liquid crystal display using the ECB system shown in FIG. 1 is driven. Although not discussed in the description given so far, a shielding film made of metal, etc., is normally formed on the opposed substrate for shutting off transmission of light except for openings 201 corresponding to the picture elements arranged like a matrix. In the shield area 200, light leakage between the picture elements is prevented and the shield area 200 is displayed in black, thereby improving display contrast. In each opening 201, the transmission rate of light is controlled to provide the desired display; a black area called disclination 202 also occurs in the opening 201. When a plurality of areas differ in orientation vector of liquid crystal, the orientation of liquid crystal directors is disarranged on boundaries between the areas, and the area indicating a transmission rate different from that in other areas is the disclination.
For liquid crystal directors in nematic phase, the orientation vector, when voltage is applied, is restricted only at an angle with the electric field direction and the azimuth with the electric field direction as an axis is released. That is, with the electric field effect only, an orientation vector oriented to a plurality of directions obtained by rotating with the electric field direction as the axis of symmetry is possible. On the other hand, the TFT substrate has electrode irregularities on the surface and surface orientation treatment is uneven. An electric field in a lateral direction exists due to the potential difference between the electrodes in a liquid crystal cell. Therefore, areas different in orientation vector of liquid crystal molecules occur in the cell. If an orientation vector error exists even partially, since liquid crystal has a continuum property, orientation vectors having an azimuth following the liquid crystal having the erroneous orientation vectors extend over a certain area. If such a phenomenon occurs at more than one place in the cell, more than one area has orientation vectors which are the same in angle with the electric field direction, but differ in azimuth. On the boundaries between the areas, the light transmission rate differs from that in other areas, causing disclination to occur. If disclination of different form for each picture element occurs frequently, the display screen will have a rough surface and the expected color display will not be provided.
If orientation vectors of liquid crystal molecules in each area become irregular in the display area, there is a chance that viewing angle dependency will arise.
On the other hand, in the VAN type, etc., due to static electricity occurring during rubbing treatment, TFT threshold or mutual conductance shift results in electrostatic discharge damage, etc.